This invention relates to a semiconductor device and, more particularly, to a semiconductor integrated circuit device with an input and output circuit.
Input signals are supplied from the outside of an integrated circuit device through the input and output circuit for pieces of information, and the integrated circuit device processes the pieces of information for producing output signals. The output signals are supplied through the input and output circuit to the outside thereof. Thus, the input and output circuit serves as an interface, and is indispensable for the integrated circuit.
A typical example of the input and output circuit is shown in FIG. 1 of the drawings. The prior art input and output circuit is incorporated in an integrated circuit 1 fabricated on a semiconductor chip, and the prior art input and output circuit includes an output buffer BF1 and an input buffer BF2. An input and output node NODE1 is shared between the output buffer BF1 and the input buffer BF2, and is connected to the output node of the output buffer BF1 and the input node of the input buffer BF2. The input node of the output buffer BF1 is connected to an internal node NODE2 of the integrated circuit, and the output node of the input buffer BF2 is connected to another internal node NODE3 of the integrated circuit.
In order to selectively activate the output/input buffers BF1/BF2, enable signals ENB1 and ENB2 are respectively supplied to the control node of the output buffer BF1 and the control node of the input buffer BF2. When the output buffer BF1 is enabled with the enable signal ENB1, the output buffer BF1 drives the input and output node NODE1 depending upon the potential level at the internal node NODE2. On the other hand, if the enable signal ENB2 is changed to the active level, the input buffer BF2 is enabled, and drives the internal node NODE3 depending upon the potential level at the input and output node NODE1. While the input buffer BF2 is driving the internal node NODE3 in response to the potential level at the input and output node NODE1, the output buffer BF1 is staying in high-impedance state, and the input and output node NODE1 is seemed to be in the high-impedance state from the point of view at the internal node NODE2.
The semiconductor integrated circuit device forms a part of an electronic system, and is mounted on a circuit board together with other semiconductor chips. A pull-down resistor 2 is also mounted on the circuit board, and is connected between the input and output node NODE1 and the ground. When the system is reset, electric charge is discharged from the input and output node NODE1 to the ground, and the input and output node NODE1 is pulled down to the ground level. The pull-down resistor 2 is desirable for the stability of the integrated circuit device. The pull-down resistor 2 may be replaced with a pull-up resistor connected between the input and output node NODE1 and a power supply line.
A problem is encountered in the prior art semiconductor integrated circuit device in that a system composer needs to connect the pull-down resistor 2 in the assembling work for the electronic system. The system composer purchases a discrete circuit component for the pull-down resistor 2, and time and labor are consumed in the assembling work. This results in increase of the production cost of the electronic system.
It is therefore an important object of the present invention to provide a semiconductor integrated circuit device, which permits a system composer to reduce the production cost of an electric system.
The present inventor contemplated the problem inherent in the prior art semiconductor integrated circuit device, and tried to integrate a pull-up/pull-down resistor on a semiconductor chip together with the integrated circuit device. The semiconductor integrated circuit device with the pull-up/pull-down resistor flew electric current at all times, and a large internal power supply circuit was required. The present inventor noticed that the input and output circuit required the pull-up/pull-down resistor only when the electric system was reset. The present inventor concluded that the input and output node was to be connected to an internal power supply line or an internal ground line at the system reset from the aspect of the power consumption.
To accomplish the object, the present invention proposes to control a switching element connected between a pull-up/pull-down resistor and a constant power line.
In accordance with one aspect of the present invention, there is provided a semiconductor integrated circuit device fabricated on a semiconductor chip comprising an interface circuit connected between a first node and a second node connectable to an external circuit for transferring a piece of information between the first node and the second node, a pull-down element connected to the second node, a switching element connected between the pull-down element and a constant power source, and responsive to a control signal at a control node thereof so as to be changed between on-state and off-state, and a controlling circuit having an input signal port connected to the first node and an output signal port connected to the control node of the switching element, and changing the control signal to an active level when the semiconductor integrated circuit device is reset and to an inactive level after the semiconductor integrated circuit restarts.
In accordance with another aspect of the present invention, there is provided a semiconductor integrated circuit device fabricated on a semiconductor chip comprising an interface circuit connected between a first node and a second node connectable to an external circuit for transferring a piece of information between the first node and the second node, a pull-up element connected to the second node, a switching element connected between the pull-up element and a constant power source, and responsive to a control signal at a control node thereof so as to be changed between on-state and off-state, and a controlling circuit having an input signal port connected to the first node and an output signal port connected to the control node of the switching element, and changing the control signal to an active level when the semiconductor integrated circuit device is reset and to an inactive level after the semiconductor integrated circuit restarts.